Electrostatic printing papers including a prime coating of a mixture of a terpolymer and an alkali metal silicate



United States Patent 3,469,977 ELECTROSTATIC PRINTING PAPERS INCLUDING A PRIME COATING OF A MIXTURE OF A TERPOLYMER AND AN ALKALI METAL SILICATE Richard L. Savage, Naugatuck, Conn., assignor to Uniroyal, Inc., New York, N.Y., a corporation of New Jersey No Drawing. Filed Oct. 1, 1965, Ser. No. 492,282 Int. Cl. G03g 7/00; B4411 1/00 U.S. Cl. 961.5 6 Claims ABSTRACT OF THE DISCLOSURE This invention is directed to an improved copy sheet for electrostatic printing. The copy sheet has a prime coating which comprises a mixture of from 30 to 70 dry parts by weight of a latex of a terpolymer, of butadiene, styrene, and an unsaturated polycarboxylic acid and from 70 to 30 dry parts by weight of an alkali metal silicate, the silicate being either potassium silicate or sodium silicate.

This invention relates to an improved paper that is for use in electrostatic printing.

Electrophotographic methods of reproduction are finding an increased use in the oflice copying field. These electrophotographic processes may be divided into general classifications of transfer photoelectrostatic or xerography and direct photoelectrostatic. In xerography, an image of the material to be copied is projected on the surface of an electrostatically charged selenium drum. The charge flows off the illuminated areas while the dark areas retain the image in latent negative electrostatic charge. The xerographic drum is then brushed with positively charged toner powder (carbon black or dye plus a thermoset resin), which adheres to the charged areas giving a visible image. A sheet of copying paper is brought against the drum to transfer the image. The toned image is fixed to the sheet by fusion of the resin binder.

In the direct photoelectrostatic process a special copying paper coated with a layer of photosensitive zinc oxide or other photosensitive material dispersed in a high dielectric adhesive is required. The sheet surface is first given a blanket negative electrostatic charge. An image is then projected on to the charged surface with the electrostatic charge being dissipated in the illuminated area. The latent image in the form of a negative charge is developed with a toner powder as in the xerographic process, or by immersion of the sheet in a dispersion of a toner in an organic carrier such as toluene. In the direct photoelectrostatic process the quality of the prints obtained is primarily dependent on the electrical properties of the copying paper. The photosensitive coating must be a high dielectric while the paper substrate must be uniformally conductive regardless of the relative humidity of the surrounding atmosphere. The base paper must also be resistant to solvent penetration to permit smooth application of the zinc oxide coating. In the process using wet toners, solvent resistance is also necessary to prevent excessive toner carrier pickup in the developing step.

An object of the present invention is to provide a prime coating composition for manufacturing electrically conductive paper for use in photoelectrostatic and electrostatic copying and reproduction processes.

A further object of the present invention is to provide an improved photosensitive coated photoelectrostatic copy paper which has a prime conductive coating.

As previously indicated, consistent results in photoelectrostatic printing are best obtained employing a copy sheet with a base that has a conductivity that does not appreciably change with changes in relative humidity. The

present invention provides such a base sheet.

highly resistant to the absorption and penetration of aromatic and aliphatic hydrocarbons. This property is necessary for the successful application of the photoconductive coating layer used in photoelectrostatic printing. The photoconductive coating usually applied to paper for such use consists of a photoconductive material dispersed in an aromatic solvent solution of a dielectric resin binder. The photoconducting layers and the methods of applying these materials are well-known in the art and are described in US. Patent 3,121,006. The present process also gives improved properties in regard to solvent holdout which is required for electrostatic printing papers which are developed with liquid toners.

The essential ingredients of the coating composition of the present invention are a latex of a synthetic rubber which contains carboxyl groups in admixture with an alkali metal silicate such as sodium or potassium silicate.

The carboxylated latex is a terpolymer of the type disclosed in Brown United States Patent 2,724,707. The preferred unsaturated carboxylic monomers of the present invention are the monomers of unsaturated polycarboxylic acids. The preferred polycarboxylic acid is itaconic acid. The ratio of monomers may vary over relatively wide ranges with the polycarboxylic acid monomer being present in amounts of from 1 to 15% by weight of all monomers in the reaction mixture. The preferred range of polycarboxylic acid monomer is from 1 to 5% by weight on the same basis. It is of course necessary that the resultant terpolymer have rubbery characteristics. In the manufacture of the carboxylated latex as disclosed by Brown, the pH of the latex is in the acid range. In order to prevent premature coagulation when the silicate of the present invention is mixed with the latex, the pH of the latex is made basic by the addition of a hydroxide such as sodium hydroxide. A final pH of from 9-11 is desired with a preferred pH of from 10-11.

The alkali metal silicates that have been found useful in the practice of the present invention include sodium silicate (NA O:SiO and potassium silicate (K O:SiO The sodium silicates that are found useful are those which have a ratio of sodium oxide to silica of from 1:4.0 to 1:16, with a ratio of 123.75 of sodium oxide to silica being the preferred ratio. Potassium silicates in the same ratio of potassium oxide to silica are also useful with a preferred ratio being 113.29 of potassium oxide to silica. The use of the sodium silicate is preferred but the us t of potassium silicate is particularly desirable if less efilorescence of the coating under high humidity conditions is required or where greater electric conductivity is required.

The ratio of carboxylated latex to alkali metal silicate may be varied over a wide range to control the hardness and flexibility of the coating film. Ratios of latex to silicate of from 30:70 to 70:30 have been found to be advantageous, with the ratio of 50:50 preferred. The use of larger amounts of silicate results in a film that is brittle and breakable. Lower amounts of silicate than indicated above make the film tacky and difiicult to process on standard equipment.

In addition to the carboxylated latex and silicate, the coating composition may contain other ingredients to modify the processing characteristics of the paper in its manufacture or the physical properties of the finished product. For example the addition of small amounts, in the range of from 2 to 20 parts per 100 parts of latexsilicate mixture, of ammonium caseinate increase the mechanical stability of the coating mix and reduce moisture sensitivity of the coating film. Other protein derivatives such as animal glue and soya protein will function in a similar manner. The addition of a tack reducer, in the amount of from 1 to 10 parts per 100 parts of latexsilicate mixture, such as a non-ionic wax emulsion, polyethylene oxide or calcium stearate is advantageous to give better paper handling properties.

Hydrogen peroxide may be added in small amounts to prevent discoloration of the final product. The usual paper coloring pigments may also be added if desired.

The present invention also finds application in paper used for electrostatic printing. In electrostatic printing, an electric charge is directly applied by electronic means to selected areas of the photosensitive dielectric material. The charged area is developed as in the photoelectrostatic process. The paper base must be conductive over wide ranges of relative humidity and must be resistant to solvent pick-up when wet toners are used.

The composition of the present invention may be applied by coating or impregnating the paper base. The pr ferred method of application is by coating each side of the base with from one-half of a pound to seven and a half pounds per three thousand square feet of paper or a total coating level of from one to fifteen pounds per three thousand square feet of paper. The preferred coating level is about eight pounds per three thousand square feet applied as a two side application. It is also possible to apply the prime coating of the present invention only to the side of the paper base that will subsequently receive the photosensitive coating.

The prime coating of the present invention forms a continuous film on the surfaces of the paper. This film is of the thickness of from about 5 to about 15 mils. In order to obtain a denser, more solvent resistant film, the prime coated paper may be super-calendered at about 120 F. to about 260 F., at nip pressures in the range of 500 to 2000 pounds per linear inch.

The prime coating composition may be applied to paper sheet with standard paper making equipment. The aqueous composition may be adjusted in solids content from 20 to 50 percent solids to meet the requirements of the particular equipment used.

The resistivity of prime coated paper should be in the range of from 1 10 to 1X10 ohms per square. As a matter of convenience, the resistivity values may be expressed as the logarithm of the measured resistivity.

In the following examples, the surface resistivity cr and the volume resistivity p of the samples were determined according to the ASTM test method D257-61 for determining electrical resistance of insulating materials. The voltage employed was 100 volts in all cases.

EXAMPLE I A latex of a butadiene-styrene-itaconic acid terpolymer was prepared employing monomers in the ratio of 44 parts butadiene, 52 parts styrene and 4 parts itaconic acid in the usual manner. The latex had a pH of 2.5. Sodium hydroxide, in an aqueous solution, was added to the latex in sufficient quantity to raise the pH to about 10.5. This latex was used as the latex portion of the coating composition in the remaining examples.

EXAMPLE II A series of mixtures of carboxylated latex and sodium silicate were prepared using the latex of Example I. The sodium silicate had a ratio of Na O:SiO of 1:3.75. The latex-silicate compositions were coated on aluminum foil of one mil thickness at a coat weight of 5 pounds per 3000 square feet per side and the volume resistivity at 4 15% and 50% relative humidity was determined. The results are as follows:

Log of volume, Resistivity Latex-silicate ratio 15% RH. 50 0 This example shows that there is relatively little variation of volume resistivity with relative humidity. The example also shows the useful ranges of latex-silicate ratios of the present invention.

EXAMPLE III Log of resistivity,

10g of surface Log of volume resistivity 0' resistivity p NaZO S10 12. 52 13.62 K20 ZSlOg 12. 23 13. 58

This example shows that potassium as well as sodium silicates gives acceptable results.

EXAMPLE IV This example shows the variation of surface and volume resistivity with the amount of coating on the base. The latex of Example I was compounded with an equal amount (dry weight) of sodium silicate, Na O:SiO ratio 1:3.75. Two parts of GUAR gulm for dry parts by weight of latex silicate mixture was added as a stabilizer. Sufiicient water was added to the mixture to give a total solids concentration of 27.5%. Samples of standard paper were then coated with different weights of the composition. The coating weights given are in pounds per ream (3000 square feet) per side of paper. The total coating weight is twice that indicated. The resistivities were determined at 5 0% relative humidity.

Coat weight per side Log of surface Log of volume (lbs.) resistivity resistivity EXAMPLE V In this example a sample of a prime coated base paper and an uncoated paper were coated with a photosensitive zinc oxide coating. The apparent surface voltage (ASV), dark decay rate and residual voltage were determined at 10% and 50% relative humidity. (RH) Regular base Prime coated base The apparent surface voltage indicates the charge acceptance of the sheet and thus its print contrast and photographic speed. The values obtained show the consistant results that may be obtained employing the present invention.

The low dark decay rate indicates that the sheet will hold its electrostatic charge while moving from the corona charging unit to the exposure area of the copying machine.

Low residual voltage after illumination is necessary for clean prints. High residual voltages cause deposition of toner in background areas on the printed sheet which results in poor contrast and poor visual quality.

EXAMPLE VI To evaluate the efiect of the conductive prime coating used in this example, both the uncoated base paper and the prime coated base paper were given a top coating of a photosensitive zinc oxide compound. This compound was prepared as follows:

80.0 grams of French Process zinc oxide (Photox 801 supplied by the New Jersey Zinc Company) was dispersed in a solution of grams of styrene-butadiene rubber (Pliolite S5B supplied by Goodyear Tire and Rubber Company) in 90 grams of toluene. The spectral sensitivity :of the zinc oxide towards ultraviolet excitation was increased by the addition of 0.005 gram of Rose Bengel dye. This coating mix was then applied to one side of the base papers at a coating weight level of 20 pounds per ream (3000 square feet).

After sensitizing the coated sheets by dark adapting them for twenty-four hours, they were run through a commercial photoelectrostatic copying machine which utilized a liquid toner development system. Due to its solvent hold-out properties the prime coated sheet of this example emerged from the machine essentially dry whereas the non-prime coated base paper adsorbed an excessive amount of toner carried and was quite damp when discharged from the copying machine. Also, due to its lower electrical conductivity, the non-prime coated sheet exhibited a tendency to reverse image on the back of the paper and to exhibit more background on the face of the sheet in the non-image areas.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. A copy sheet for use in receiving an electrostatic latent image comprising a paper base sheet, a prime coating in an amount of from one-half to seven and one-half pounds per three thousand square feet on each side of said base sheet, said prime coating comprising a mixture of from 30 to dry parts by weight of a latex of a terpolymer of styrene, butadiene and an unsaturated polycarboxylic acid and from 70 to 30 dry parts by weight of an alkali metal silicate in which the ratio of the alkali metal oxide to silica is from 1:4 to 121.6, and a photoconductive coating on said prime coating.

2. The copy sheet of claim 1 in which the unsaturated polycarboxylic acid in the terpolymer is itaconic acid and the alkali metal silicate is selected from the group consisting of potassium silicate and sodium silicate.

3. The copy sheet of claim 2 in which the alkali metal silicate is sodium silicate and the ratio of sodium oxide to silica is 13.75.

4. The copy sheet of claim 3 in which the ratio of latex to sodium silicate in the prime coating composition is 50:50 on a dry weight basis.

5. The copy sheet of claim 4 in which the coat weight is five pounds per 3000 square feet per side.

6. The copy sheet of claim 2 in which the prime coating is from 1 to 15 mils thick and the sheet has a surface resistivity of from 1X 10 to 1x10 ohms per square at a potential of 100 volts.

References Cited UNITED STATES PATENTS 2,724,707 11/1955 Brown 260-] 3,295,967 l/1967 Schoenfeld 961.S

NORMAN G. TORCHIN, Primary Examiner I. C. COOPER, Assistant Examiner US. Cl. X.R. -l.8; 117-218 

